Among hydroxybenzoic acids, o-hydroxybenzoic acid (e.g., salicylic acid) is used as a dye or an intermediate thereof, an antiseptic agent, a stock agent, a keratin solubilizor, a dermatitis therapy and so forth. In particular, an acetylsalicylic acid (aspirin), a derivative of salicylic acid, is a very important compound as an antipyrin, an analgesic and a resolution agent. P-hydroxybenzoic acid is employed as a raw material of polyesters and an ester derivative thereof is used as an antibacteria agent or a stock agent.
O-hydroxybenzoic acid or p-hydroxybenzoic acid is usually produced by Kolbe-Schmit reaction using phenol and sodium hydroxide or potassium hydroxide via sodium hydroxybenzoate or potassium hydroxybenzoate. Since the hydroxybenzoate is industrially converted to an hydroxybenzoic acid having a free hydroxyl group by an acid such as sulfuric acid, a treatment of metal salts (e.g., sodium sulfonate, potassium sulfonate) produced as by-products is required.
Japanese Patent Application Laid-open No. 38909/1996 (JP-A-8-38909) provides an oxidation process using an imide compound (e.g., N-hydroxyphthalimide) as a catalyst at a stage of oxidation of a substrate (e.g., hydrocarbon) with oxygen. When this oxidation process is applied to an aromatic compound having a methyl group, aromatic compounds having a carboxyl group may be formed. However, when a compound such as cresol is used, it is difficult to obtain an hydroxybenzoic acid.